These changes allow to set FQDN as hostname of pods for pods
that set the new PodSpec field setHostnameAsFQDN to true. The PodSpec
new field was added in related PR.
This is PART2 (last) of the changes to enable KEP #1797 and addresses #91036
Users must not be allowed to step outside the volume with subPath.
Therefore the final subPath directory must be "locked" somehow
and checked if it's inside volume.
On Windows, we lock the directories. On Linux, we bind-mount the final
subPath into /var/lib/kubelet/pods/<uid>/volume-subpaths/<container name>/<subPathName>,
it can't be changed to symlink user once it's bind-mounted.
